An information recording and reproducing system has recently been proposed in which pieces of information are in a thermomagnetic fashion written in and read out from a magneto-optic information storage medium consisting of a film of an amorphous alloy of, for example, gadolinium iron (GdFe) or gadolinium cobalt (GdCo). A thin film of such an amorphous alloy is magnetizable in directions normal to the surfaces of the film and is operable as a normal magnetization film. When the normal magnetization film is heated to a temperature higher than the Curie point or the magnetic compensation temperature of the alloy with a weak bias field applied perpendicularly to the normal magnetization film, re-orientation of the spins of electrons takes place and as a consequence the direction of magnetization is inverted in the zone subjected to the bias field. If the film is preliminarily magnetized in one sense normal to the surfaces of the film and is then irradiated with a recording beam of laser at selected elementary spots of the film, the normal magnetization film is thus heated to such a temperature at the irradiated spots thereof with the result that the direction of magnetization at the particular spots is inverted in the presence of a weak bias field applied in the opposite sense to the surfaces of the film. If the original direction of magnetization is assumed to correspond to a logic "0" binary signal, then the inverted direction of magnetization will correspond to a logic "1" binary signal. Logic "0" and "1" binary signals can thus be recorded on a normal magnetization film preliminarily magnetized in a direction representative of either logic "0" or "1" binary signal when the film is irradiated with a laser beam at selected elementary spots thereof in the presence of a bias magnetic field.
The pieces of information which have in this fashion been stored on a magneto-optic information storage medium are read out from the storage medium through detection of the directions of magnetization at the individual elementary spots of the normal magnetization film by a method utilizing a magneto-optic phenomenon known as the magnetic Kerr effect. For this purpose, the normal magnetization film having the pieces of information stored thereon is irradiated with a recording beam of laser passed through a polarizer and the resultant linearly polarized laser beam is focused to the individual elementary spots of the normal magnetization film. The linearly polarized light thus incident on an elementary spot of the normal magnetization film is caused to have its plane of polarization turned through a certain angle in a direction which depends on the direction of magnetization at the particular spot. The laser beam reflected from the normal magnetization film is directed to an optic analyzer which transmits therethrough only the linearly polarized light reflected from an elementary spot magnetized in one direction. The linearly polarized light reflected from an elementary spot magnetized in the other direction is prohibited from being transmitted through the analyzer. The directions of magnetization at the individual elementary spots of the normal magnetization film can be in this manner detected from the beams of light emanating from the analyzer.
A magneto-optic information storage medium of the above described nature is operable for the recording of colorplexed composite video signals. For this purpose, frequency modulated versions of original composite video signals might be recorded on the magneto-optic medium. The frequencies of frequency modulated colorplexed composite video signals range up to an extremtly high level and, for this reason, an information storage medium to record such signals is required to have frequency characteristics higher than 5 megahertz. Such high frequency characteristics are not available with a magneto-optic information storage using the magnetic Kerr effect and, therefore, a magneto-optic information storage medium using a normal magnetization film is practically not operable for the recording of colorplexed composite video signals in a known magneto-optic information recording system.
It is, accordingly, an important object of the present invention to provide a magneto-optic information recording system which will make it possible to record colorplexed composite video signals on a magneto-optic information storage medium using a normal magnetization film without frequency modulating the original video signals.
It is another important object of the present invention to provide a combination of such an information recording system and a magneto-optic information reproducing system for thermomagnetically reproducing the information recorded on an information storage medium using a normal magnetization film.